为什么VC ++字符串不引用计数？ [英] Why VC++ Strings are not reference counted?

问题描述

STL标准不需要从std :: string引用。但实际上大多数C ++
实现提供了引用计数的写时复制字符串，允许您将
string by value作为基本类型传递。这些实现（至少g ++）使用
原子操作使这些字符串无锁和线程安全。

轻松测试显示copy-on-write语义：

  #include< iostream> 
 #include< string> 
 
 using namespace std; 
 
 void foo（string s）
 {
 cout<<（void *）s.c_str（）<< endl; 
 string ss = s; 
 cout<<（void *）ss.c_str（）<<< endl; 
 char p = ss [0]; 
 cout<<（void *）ss.c_str（）<<< endl; 
} 
 
 int main（）
 {
 string s =coocko; 
 cout<<（void *）s.c_str（）<< endl; 
 foo（s）; 
 cout<<（void *）s.c_str（）<< endl; 
} 
  

在使用非常数成员后，只打印两个地址。 / p>

我使用HP，GCC和Intel编译器测试了此代码，并得到类似的结果 - 字符串
用作写入时复制容器。

另一方面，VC ++ 2005清楚地显示每个字符串都完全复制。

为什么？

我知道VC ++ 6.0中有一个错误，它有非线程安全的
引用计数引起的随机程序崩溃。这是原因吗？他们只是
害怕使用ref计数，甚至是常见的做法吗？

感谢

解决方案

我认为越来越多的 std :: string 实现将远离refcounting / copy-on-write，因为它通常是

请参阅Herb Sutter的文章未进行优化（在多线程世界中）。

STL standard do not require from std::string to be refcounted. But in fact most of C++ implementations provide refcounted, copy-on-write strings, allowing you passing string by value as a primitive type. Also these implementations (at least g++) use atomic operations making these string lock-free and thread safe.

Easy test shows copy-on-write semantics:

#include <iostream>
#include <string>

using namespace std;

void foo(string s)
{
	cout<<(void*)s.c_str()<<endl;
	string ss=s;
	cout<<(void*)ss.c_str()<<endl;
	char p=ss[0];
	cout<<(void*)ss.c_str()<<endl;
}

int main()
{
	string s="coocko";
	cout<<(void*)s.c_str()<<endl;
	foo(s);
	cout<<(void*)s.c_str()<<endl;
}

Only two adresses are printed exactly after a non-constant member was used.

I tested this code using HP, GCC and Intel compiler and got similar results -- strings work as copy-on-write containers.

On the other hand, VC++ 2005 shows clearly that each string is fully copied.

Why?

I know that there was a bug in VC++6.0 that had non-thread-safe implementation of reference counting that caused random program craches. Is this the reason? They just afraid to use ref-counting any more even it is common practice? They prefer to not use ref-counting at all over fixing the issue?

Thanks

解决方案

I think that more and more std::string implementations will move away from refcounting/copy-on-write as it is often a counter-optimization in multi-threaded code.

See Herb Sutter's article Optimizations That Aren't (In a Multithreaded World).

这篇关于为什么VC ++字符串不引用计数？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！